Over 60,000 species occur worldwide, with about 3,000 in North America - more than any other Hymenoptera family. The distribution of Ichneumonidae has traditionally been one of the most notable exceptions to the common latitudinal gradient in species diversity, since the family is thought to be at its most species rich in high latitudes instead of the tropics.[1] This traditional view has recently been questioned after the discovery of numerous new tropical species.[2][3]

Ichneumon wasps differ from typical, aculeate wasps, which sting in defense and do not pass their eggs along the stinger (Aculeata: Vespoidea and Apoidea), in that the antennae have more segments; typically 16 or more, whereas the others have 13 or fewer. Female ichneumon wasps sometimes have an ovipositor longer than their body. Ovipositors and stingers are homologous structures; Ichneumons generally inject venom along with the egg, but only larger species, with relatively shorter ovipositors, use the ovipositor as a stinger in defense. Stingers in aculeate Hymenoptera - which like Ichneumonidae belong to the Apocrita - are used exclusively for defense; they cannot be used as egg-laying equipment. Males do not possess stingers or ovipositors in either lineage.

Ichneumonidae are distinguished from their sister groupBraconidae by these character combinations. Vein 2m-cu of the forewing is present in 95% of Ichneumonidae - it is absent in Braconidae; vein 1/Rs+M is absent in all Ichneumonidae- it is present in 85% of Braconidae. Vein 1R-M of the hind wing is opposite or apical to the separation of R1 and Rs in Ichneumonidae- it is basal in Braconidae. About 90% of Ichneumonidae have a flexible suture between metasomal terga 2 and 3 -these tergites are fused in Braconidae (though the suture is secondarily flexible in Aphidiinae).[4]

Some species of ichneumon wasps lay their eggs in the ground, but most inject them directly into a host's body, typically into a larva or pupa. Host information has been notably summed up by J.F. Aubert, et al.[5]

In some of the largest species, namely from the genera Megarhyssa and Rhyssa, both sexes will wander over the surfaces of logs and tree trunks, tapping with their antennae. Each sex does so for a different reason; females are searching for the scent of wood-boring larvae of the horntail wasps (hymenopteran family Siricidae) upon which to lay eggs; males are searching for emerging females with which to mate.

Upon sensing the vibrations emitted by a wood-boring host, the female wasp will drill her ovipositor into the substrate until it reaches the cavity wherein lies the host. She then injects an egg through the hollow tube into the body cavity. There, the egg will hatch and the resulting larva will devour its host before emergence. How a female is able to drill with her ovipositor into solid wood is still somewhat of a mystery, though metal (ionized manganese or zinc) is found in the extreme tip of some species' ovipositors. The adult insect, following pupation, is faced with the problem of extricating itself from tunnels of its host. Fortunately, the high metal concentrations are not limited to the female's ovipositor, as the mandibles of the adult are also hardened with metals and it uses these to chew itself out of the wood.[6]

The Ichneumonidae are the subject of ongoing taxonomical difficulty. About as diverse as the true weevils (Curculionidae), there are numerous small, inconspicuous and hard-to-identify ichneumon wasps. The sheer diversity means DNA sequence data are only available for a tiny fraction of the species, and detailed cladistic studies require major-scale computing capacity.

Regardless, a number of seminal works exist, including the extensive study and the synonymic catalogue by Townes, but also treatments by other entomologists, namely J. F. Aubert, whose notable collection of ichneumon wasps is in Lausanne.[8]

The list presented here follows the suggestion of David Wahl of the American Entomological Institute.[9] It will be updated as necessary, as new research refines the interrelationships of the ichneumon wasps.

The subfamilies are not listed in a taxonomic or phylogenetic sequence, as the relationships between the groups are not yet resolved to a degree to render any such arrangement reliable:[9]

The grisliness and apparent cruelty (at least, from a human perspective) of Ichneumonidae larval cannibalism troubled philosophers, naturalists, and theologians in the 19th century, who found the practice inconsistent with the notion of a world created by a loving and benevolent God.[11]Charles Darwin found the example of the Ichneumonidae so troubling, it contributed to his increasing doubts about the nature and existence of a Creator. In an 1860 letter to the American naturalist Asa Gray, Darwin wrote:

I own that I cannot see as plainly as others do, and as I should wish to do, evidence of design and beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice.[12]